Swell and Swell Pressure Tests

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    E M 1410-2-1906

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    1 May 80

    APPENDIX VIIIA:

    SWELL AND SWELL PRESSURE TESTS

    INTRODUCTION. Swell is the process of imbibing available mois-

    ture to cause an increase in soil volume until the pore water pressure

    increases to an equilibrium determined by the environment. The

    amount of swell to satisfy the new pore pressure equilibrium depends

    on the magnitude of the vertical loading and soil properties that in-

    clude soil composition, natural water content and density, and soil

    structure. The rate of swell depends on the coefficient of perme-

    ability (hydraulic conductivity), thickness, and soil properties. Soils

    that are more likely to swell appreciably include clays and clay

    shales with plasticity indices greater than 25, liquid limits greater

    than 40, and natural water contents near the plastic limit or less.

    The presence of capillary stress or negative pore water pressure

    arising from molecular forces in swelling soils causes available

    moisture to be absorbed. The vertical confining pressure required to

    prevent volume expansion from absorbed moisture is defined as the

    swell pressure.

    The swell and swell pressure are generally determined in the

    laboratory with the one-dimensional consolidometer (Appendix VIII,

    CONSO LIDAT ION TEST) . Swell is determined by subjecting the

    laterally confined soil specimen to a constant vertical pressure and

    by giving both the top and bottom of the specimen access to freewater (usually distilled) to cause swell. The swell pressure is

    determined by subjecting the laterally confined soil specimen to in-

    creasing vertical pressures, following inundation, to prevent swell.

    2 . APPARATUS, CALIBRATION OF EQUIPMENT, AND PREPARA-

    TION OF SPECIMENS. The apparatus is essentially the same as that

    listed in paragraph 2, Appendix VIII, CONSOLIDATION TEST.

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    80Smoothly ground porous stones should be used in the consolidometer

    to minimize seating displacements. Filter paper should not be used

    because it is compressible and contributes to displacements. The

    equipment is calibrated and the sample prepared in the same manner

    as described for the consolidation test.

    3 . PRO C ED U RE.

    2. Swell Test. 1 Record all identifying information for the

    specimen, such as project number, boring number, and other pertinent

    data, on the data sheet (Plate VIII-J, pVIII-18,

    is a suggested form);

    note any difficulties encountered in preparation of the specimen.Measure and record the height and cross-sectional area of the

    speci men. Record weight of specimen ring and glass plates. After

    the specimen is prepared, record the weight of the specimen plus

    tare (ring and glass plates), and from the soil trimmings obtain 200 g

    of material for specific gravity and water content determinations.

    Record the weight of the material used for the water content deter-

    mination on the data sheet.

    (2) Fit an air-dried, smoothly ground porous stone into

    the base of a dry consolidometer. Place the ring with the specimen

    therein on top of the porous stone. If the fixed-ring consolidometer

    is used, secure the ring to the base by means of clamps and screws.

    (3) Place the top air-dried, smoothly ground porous stone

    and loading plate in position. The inside of the reservoir should be

    moistened to promote a high-humidity environment. The reservoir

    and loading plate should subsequently be covered with a sheet of

    impervious material such as plastic film or moist paper towel to

    inhibit loss of moisture.

    (4) Place the consolidometer containing the specimen in

    the loading device.

    (5) Attach the dial indicator support to the consolidometer,

    and adjust it so that the stem of the dial indicator is centered with

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    respect to the specimen. Adjust the dial indicator to allow for both

    swell and consolidation measurements.

    (6) Adjust the loading device until it just makes contact

    with the specimen. The seating load should not exceed about 0.01 ton

    per sq ft.

    (7) Read the dial indicator and record the reading on a data

    sheet (Plate VIII-2, p VIII-i9, is a suggested form). This is the

    initial reading of the dial indicator.

    (8) Depending on the particular design considerations, a

    specific load (e.g. overburden plus design load) is applied to the speci-

    men. After a period of at least min but less than 30 min (to avoid

    shrinkage from drying), record the dial reading on the data sheet

    (Plate VIII-2) and inundate the specimen.

    (9) Inundate the specimen by filling the reservoir, within the

    inundation ring that surrounds the specimen, with water (distilled,

    tap, or field pore water, actual or reconstituted). Cover with the

    plastic films and moist paper towel or equivalent. If a fixed-ring

    device is used, a low head of water should be applied to the base

    of the specimen and maintained during the test by means of the

    s andpipe.

    40 Observe and record on the data sheet (Plate VIII-2 the

    deformation as determined from dial indicator readings after various

    elapsed times. Readings at 0.1, 0.2, 0.5, 1.0, 2.0, 4.0, 8.0, 15.0, and

    30.0 min, and 1, 2, 4, 8, 24, 48, and 72 hr are usually satisfactory.

    A timing device should be located near the consolidometer to ensure

    accurately timed measurements. Allow the load increment to remain

    on the specimen until it is determined that the primary swell is com-

    pleted. Time to complete the primary swell of heavy clay soils and

    clay shales often requires three or more days. Plot the dial reading

    versus time data on a semilogarithmic plot as shown in Plate VIIIA-1

    to ascertain the completion of primary swell. The completion of

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    1 Mayprimary swell is arbitrarily defined as the intersection of the tangent

    to the curve at the point of inflection, with the tangent to the straight-

    line portion representing a secondary time effect as shown in Figure i.

    This is similar to the procedure in paragraph 5j, Appendix VIII,

    CONSOLIDATION TEST.

    ii Although a falling-head permeability test may be per-

    formed at this point of the swell test (see paragraph 8, Appendix VII,

    PERMEABILITY TESTS), the specimen may not be fully saturated and

    the permeability results consequently affected. A.fter primary swell

    iscomplete,t

    the load should be removed in decrements according to

    the procedure in paragraphs 51 and Appendix VIII, CONSOLIDA-

    TION TEST. The final load should be the seating load.

    b. Swell Pressure Test.

    The procedure of this test is identical

    with the preceding swell test through (9). Following 9 , increments

    of load are applied as needed to prevent swell. Variations from the

    dial reading at the time the specimen is inundated with water should

    be kept preferably within 0.0002 in. and not more than 0.0005 in.

    Following 24 hr after the specimen exhibits no further tendency to

    swell, a falling-head permeability test may be performed (see para-

    graph 8, Appendix VII, PERMEABILITY TESTS) and the final load

    (which is also the swell pressure) should be removed in decrements

    according to the procedure in paragraphs 52 and 5m, Appendix VIII,

    CONSOLIDATION TEST. The final load should be the seating load.

    4. COMPUTATIONS. The computations for the swell tests are similar

    to those presented in paragraph 6, Appendix VIII, CONSOLIDATION TEST.

    After primary swell is complete, the loading pressure may be in-

    creased to consolidate the specimen until the void ratio is less than

    the initial void ratio under the overburden pressure; thereafter,

    loads may subsequently be removed to determine rebound charac-

    te ri st ic s. This procedure permits an alternative measure of the

    swell pressure, defined as the total pressure required to reduce

    the void ratio to the initial void ratio.

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    and optimum water content.

    6. POSSIBLE ERRORS. In addition to the possible errors discussed

    in paragraph 8, Appendix VIII, CONSOLIDATION TEST, the following

    may also cause inaccurate determination of swelling characteristics:

    a. Displacements caused by seating of the specimen against

    the surface of the porous stones may be significant, especially if

    swell displacements and loading pressures are small. Thus, smoothly

    ground porous stones are recommended.

    k. Filter paper is highly compressible and contributes to the

    observed displacements and hysteresis in displacements on loading

    and .rebound. Filter paper is consequently not recommended.

    c. The compressibility characteristics of the consolidometer

    and the test procedures influence the swell pressure results. Because

    very small expansions in volume greatly relieve swell pressures, the

    stiffness of the consolidometer should be as large as possible, and

    variations in displacements that occur during determinaiion of the

    swell pressure should be as small as possible.

    d. Swelling char.acteristics determined by consolidometer swell

    tests for the purpose of predicting heave of foundation and compacted

    soils are not representations of many field conditions because:

    1 Lateral swell and lateral confining pressures are not

    simulated.

    (2) The actual availability of water to the foundation soils

    may be cyclic or intermittent. Field swell usually occurs under

    constant overburden pressure depending on the availability of water.

    The swell index, in contrast, is evaluated by observing swell due to

    decreases in overburden pressure while the soil specimen is inun-

    dated with water.

    (3) Rates of swell indicated by swell tests are not reliable

    indicators of field rates of swell due to fissures in the mass soil and

    inadequate simulation of the actual availability of water to the soil.

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    (4) Secondary or long-term swell, which is not evaluated by

    these test procedures, may be significant for some clays and clay

    shales . These soils may not be fully saturated at the conclusion of

    the swell test.

    (5) Chemical content of the inundating water affects results;

    e.g., when testing shales, distilled water may give radically different

    results than natural or reconstituted pore water.

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